Method of producing a semiconductor laser and semiconductor laser

ABSTRACT

A semiconductor laser includes a substrate having a semiconductor layer sequence with an active layer that generates light during operation of the semiconductor laser, and a contact layer on a bottom side of the substrate opposite the semiconductor layer sequence, wherein the contact layer has at least one first partial region and at least one second partial region which are formed contiguously, the at least one first partial region is annealed, and the at least one second partial region is unannealed.

TECHNICAL FIELD

This disclosure relates to a method of manufacturing a semiconductorlaser and a semiconductor laser.

BACKGROUND

Laser diodes in the spectral range from ultraviolet to infrared areincreasingly opening up new markets, for example, in the area oflighting, projection and material processing applications, where theiradvantages over light-emitting diodes, for example, in terms ofincreased luminance come into play. Such laser diodes are usually basedsubstantially on epitaxial structures with as few defects as possible,for example, on GaN or GaAs substrates, where the n-contact is appliedto the rear side of the substrate. As experiments have shown, asignificant voltage drop contribution appears at this n-contact, whichcan be problematic as the efficiency of the laser diodes can be reducedand thus the component stability can also be negatively impaired.

SUMMARY

We provide a method of manufacturing a semiconductor laser includingproviding a substrate having a semiconductor layer sequence with anactive layer that generates light during operation of the semiconductorlaser, applying a continuous contact layer having at least one firstpartial region and at least one second partial region on a bottom sideof the substrate opposite the semiconductor layer sequence, and locallyannealing the contact layer only in the at least one first partialregion.

We also provide a semiconductor laser including a substrate having asemiconductor layer sequence with an active layer that generates lightduring operation of the semiconductor laser, and a contact layer on abottom side of the substrate opposite the semiconductor layer sequence,and wherein the contact layer has at least one first partial region andat least one second partial region which are formed contiguously, andthe at least one first partial region is annealed and the at least onesecond partial region is unannealed.

We further provide a method of manufacturing a semiconductor laserincluding providing a substrate having a semiconductor layer sequencewith an active layer that generates light during operation of thesemiconductor laser, applying a continuous contact layer having at leastone first partial region and at least one second partial region on abottom side of the substrate opposite the semiconductor layer sequence,and locally annealing the contact layer only in the at least one firstpartial region, wherein each second partial region remains unannealed inthe finished semiconductor laser.

We still further provide a semiconductor laser including a substratehaving a semiconductor layer sequence with an active layer thatgenerates light during operation of the semiconductor laser, and acontact layer on a bottom side of the substrate opposite thesemiconductor layer sequence, wherein the contact layer has at least onefirst partial region and at least one second partial region which areformed contiguously, the at least one first partial region is annealed,and the at least one second partial region is unannealed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E show schematic illustrations of a method of manufacturinga semiconductor laser according to an example.

FIGS. 2A to 9C show schematic illustrations of semiconductor lasersaccording to further examples.

FIGS. 10A to 10D show schematic illustrations of semiconductor lasersaccording to further examples.

FIGS. 11A to 11C show schematic illustrations of semiconductor lasersaccording to further examples.

FIGS. 12A to 12C show images of sections of semiconductor lasersaccording to further examples.

REFERENCE LIST

1 substrate

2 semiconductor layer sequence

3 active layer

4 electrode layer

5 active area

6 light outcoupling surface

7 rear surface

8 light

9 ridge waveguide structure

10 bottom side

11 contact layer

12 first partial region

13 second partial region

14 layer

20 surface structure

19 passivation layer

90 irradiation

100 semiconductor laser diode

DETAILED DESCRIPTION

In our method of manufacturing a semiconductor laser diode, an activelayer may be provided, which is embodied and intended to generate lightduring operation of the semiconductor laser diode. A semiconductor laserdiode may have at least one active layer embodied and intended togenerate light in an active region during operation. The examples andfeatures described below apply equally to the semiconductor laser diodeand to the method of manufacturing the semiconductor laser diode.

In particular, the active layer can be part of a semiconductor layersequence with a plurality of semiconductor layers. For example, theactive layer can have exactly one active area by which laser light canbe emitted during operation. The active region can be defined at leastpartially by a contact surface of the semiconductor layer sequence withan electrode layer on the semiconductor layer sequence, i.e. at leastpartially by a surface over which current is applied to thesemiconductor layer sequence and thus into the active layer.Furthermore, the active area can at least partially be defined by aridge waveguide structure, i.e. a ridge formed in the form of anelongated elevation in the semiconductor material of the semiconductorlayer sequence. Furthermore, the active layer can also have a pluralityof active areas that can be formed by a corresponding plurality of oneor more of the described measures.

The semiconductor layer sequence can in particular be an epitaxial layersequence, i.e. an epitaxially grown semiconductor layer sequence. Thesemiconductor layer sequence can be based on InAlGaN, for example.InAlGaN-based semiconductor layer sequences include in particular thosein which the epitaxially produced semiconductor layer sequence generallycomprises a layer sequence of different individual layers containing atleast one individual layer comprising a material from the III-V compoundsemiconductor material system In_(x)Al_(y)Ga_(1-x-y)N-with 0≤x≤1, 0≤y≤1and x+y≤1. In particular, the active layer can be based on such amaterial. Semiconductor layer sequences that have at least one activelayer based on InAlGaN can, for example, emit electromagnetic radiationin an ultraviolet to green wavelength range. Alternatively oradditionally, the semiconductor layer sequence can also be based onInAlGaP, i.e. the semiconductor layer sequence can have differentindividual layers, of which at least one individual layer, e.g. theactive layer, comprises a material made of the III-V compoundsemiconductor material system In_(x)Al_(y)Ga_(1-x-y)P with 0≤x≤1, 0≤y≤1and x+y≤1. Semiconductor layer sequences having at least one activelayer based on InAlGaP can, for example, preferably emit electromagneticradiation with one or more spectral components in a green to redwavelength range. Alternatively or additionally, the semiconductor layersequence may also comprise other III-V compound semiconductor materialsystems such as an InAlGaAs-based material or II-VI compoundsemiconductor material systems. In particular, an active layer of asemiconductor laser comprising an InAlGaAs based material may be capableof emitting electromagnetic radiation having one or more spectralcomponents in a red to infrared wavelength range. A II-VI compoundsemiconductor material may have at least one element from the secondmain group such as Be, Mg, Ca, Sr, and one element from the sixth maingroup such as O, S, Se. For example, the II-VI compound semiconductormaterials include ZnO, ZnMgO, CdS, ZnCdS and MgBeO.

The active layer and, in particular, the semiconductor layer sequencewith the active layer can be arranged on a substrate. In particular, thesubstrate with the semiconductor layer sequence with the active layercan be provided. For example, the substrate can be a growth substrate onwhich the semiconductor layer sequence is grown. The active layer and,in particular, a semiconductor layer sequence with the active layer canbe grown by an epitaxial process, for example, by metal-organic vaporphase epitaxy (MOVPE) or molecular beam epitaxy (MBE), on the growthsubstrate formed as a wafer and, furthermore, provided with electricalcontacts. Furthermore, it may also be possible that the growth substrateis removed after the growth process. In this example, the semiconductorlayer sequence can also be transferred to a substrate embodied as acarrier substrate after growth. The substrate may comprise asemiconductor material such as a compound semiconductor material systemmentioned above or other material. In particular, the substrate can beelectrically conductive and, for example, contain Ga. In this example,the substrate may contain or be made of GaAs, GaP and/or GaN.Alternatively or additionally, the substrate can also comprise InP, SiC,Si and/or Ge or be made of such a material.

For example, the active layer may comprise a conventional pn-junction, adouble heterostructure, a single quantum well structure (SQW structure),or a multiple quantum well structure (MQW structure) for lightgeneration. In addition to the active layer, the semiconductor layersequence can also comprise other functional layers and functionalregions such as p- or n-doped charge carrier transport layers, i.e.electron or hole transport layers, undoped or p- or n-doped confinement,cladding or waveguide layers, barrier layers, planarization layers,buffer layers, protective layers and/or electrodes as well ascombinations thereof. Moreover, additional layers such as buffer layers,barrier layers and/or protective layers can also be arrangedperpendicular to the growth direction of the semiconductor layersequence, for example, around the semiconductor layer sequence, i.e. onthe side surfaces of the semiconductor layer sequence.

A contact layer may be applied to the bottom side of the substrateopposite the semiconductor layer sequence. Together with the electrodelayer on the semiconductor layer sequence described above, by which,depending on its realization, an active area can be defined the contactlayer applies current to the semiconductor layer sequence and, inparticular, into the active layer during operation of the semiconductorlaser. The side on which the electrode layer is applied can also bereferred to as the epitaxial side, while the side on which the contactlayer is applied can be referred to as the substrate side. The contactlayer has at least one first partial region and at least one secondpartial region that are formed contiguously. In other words, the atleast one first partial region and the at least one second partialregion form a continuous part of the contact layer and can in particulardirectly adjoin each other in a lateral direction. A lateral directionis defined as a direction parallel to the bottom side of the substrateand thus preferably parallel to the main plane of the active layer andthe other semiconductor layers of the semiconductor layer sequence. Thatthe contact layer comprises at least one first and at least one secondpartial region means that the contact layer comprises one or a pluralityof first partial regions and one or a plurality of second partialregions, all of which are laterally contiguous and together form alayer. In particular, each of the first partial regions may be directlyadjacent to at least one second partial region and vice versa. As aresult, the contact layer may have a plurality of first partial regionsseparated from each other by one or more second partial regions, whichare tempered locally. The at least one first partial region may have oneor more geometric shapes selected from: line, cross, circle, ellipse,spiral, grid, square, wavy line.

The contact layer may be annealed only in the at least one first partialregion. In particular, this may mean that the contact layer is notannealed in any of the one or more second partial regions, but onlylocally in the one or more first partial regions of the contact layer.The annealing can be caused by an increase in temperature locally in theat least one first partial region. By the annealing, a mixing ofmaterials of the contact layer can occur in the at least one firstpartial region. Furthermore, a mixing of materials of the contact layerand materials of the substrate can occur. The annealing can thus causethe contact layer to be alloyed into at least the first partial region.In contrast, the contact layer can remain unchanged in the at least onesecond partial region so that no material mixing and thus no alloyingtakes place in the at least one second partial region. If the contactlayer has a layer structure in the form of a layer stack, this structurecan be retained in the at least one second partial region, while thisstructure is altered in the at least one first partial region by thealloying. The semiconductor laser can thus have a continuous contactlayer with at least one first partial region and at least one secondpartial region on a bottom side of the substrate opposite thesemiconductor layer sequence, wherein the at least one first partialregion is annealed and the at least one second partial region isunannealed. The at least one second partial region remains unannealed,especially in the finished semiconductor laser. This also applies toeach of the second partial regions in a contact layer with a pluralityof second partial regions.

After growing in the form of a wafer on a substrate, the semiconductorlayer sequence may first be processed on the side facing away from thesubstrate, which can also be referred to as the epitaxial side comparedto the substrate side as described above. The epitaxial side maypreferably be the p-side of the semiconductor layer sequence, while thesubstrate side may be the n-side of the semiconductor layer sequence.Alternatively, the polarity of the semiconductor layer sequence can alsobe reversed. When processing the epitaxial side, the active regions of alarge number of semiconductor lasers are usually generated, for example,by the production of ridge waveguide structures, which then receive alateral dielectric passivation. The electrode layer for contacting theepitaxial side is deposited on top thereof. Subsequently, the substratecan be thinned to achieve improved facet breaking and/or reducedoperating voltage during later operation of the semiconductor lasers.The contact layer can then be applied to the bottom side of thesubstrate and annealed locally in the first areas. After applying thecontact layer on the substrate side, the wafer process can be completed.By subsequent breaking of the laser facets, laser bars with a largenumber of active areas can be produced. Then, the facets can be providedwith mirror properties. Furthermore, a laser bar can be separated intolaser bars with less active areas or into individual lasers. Thesemiconductor laser described here can thus be a single laser with oneactive region or a laser bar with a plurality of active regions. Inprocess steps carried out in a wafer composite, the term semiconductorlaser can also apply to the areas of the wafer corresponding to thelater singulated semiconductor lasers.

In laser diodes in which the contact on the substrate side is notthermally alloyed, the contact shows only inadequate ohmic behavior.However, such an alloying is often avoided in the state of the art sincethe previously applied epitaxial-side electrode layer would sufferelectrical voltage losses at typical temperature conditions of 250° C.to 500° C., depending on the material system and the contact materials.In addition, at the elevated temperatures of the alloying process, amixing of the contact materials typically applied in layer stacks wouldoccur. Furthermore, in a Ga-containing substrate, for example, galliumfrom the substrate as well as titanium, when it is used, for example, asan adhesion promoter layer of a contact, could partially reach thesurface of the component and could, via corresponding oxide formation,impair or even prevent bond wire bonding or soldering of the componentduring the assembly process.

A potential way of avoiding damage to the epitaxial contact whenalloying the substrate contact is to reverse part of the processsequence by applying the epitaxial contact only after alloying thesubstrate contact. However, this process sequence requires the wafer bethinned before the long process sequence on the epitaxial side such asactivation, fabrication of ridge waveguide structures, passivation,metallization and/or mesa etching. Thus, this reverse process sequencehas the significant disadvantage that most technological steps providean increased risk of breakage due to the thinned wafer so that highyield losses are to be expected.

The local annealing may be carried out by irradiation. In particular,the contact layer may be irradiated from the side facing away from thesubstrate. In particular, a laser-based irradiation method can be usedas the irradiation method. A laser can therefore be used to irradiatethe contact layer and the bottom side of the substrate from the side ofthe contact layer facing away from the substrate. The selectiveirradiation of the contact layer in the at least one first partialregion can be achieved by using a laser so that a local heating and thusa local annealing can be achieved. Selective irradiation can be carriedout by scanning so that local heating can be targeted with regard to thesize, shape and number of the first partial regions. For effectiveheating, laser light in particular can be used, which is at leastpartially absorbed by the substrate. Alternatively or additionally,laser light can be used which is at least partially absorbed by one ormore materials of the contact layer. As an alternative to a laser, itmay also be possible to use a different light source such as focusedlight from one or more light emitting diodes and/or halogen lamps and/orgas discharge lamps.

The contact layer may be applied over the entire surface on the bottomside of the substrate. This can mean in particular that thesemiconductor laser has a contact layer that completely covers thebottom side of the substrate.

The contact layer may be applied such that an edge region of the bottomside of the substrate of the semiconductor laser is free of the contactlayer. In particular, a circumferential edge region of the bottom sideof the substrate of the semiconductor laser can be free of the contactlayer so that the contact layer does not reach the substrate edge in thelateral direction in a circumferential region. A correspondingstructuring of the contact layer can already take place in the wafercompound, i.e. before separation into individual semiconductor lasers,whereby each area corresponding to a later separated semiconductor laserhas a corresponding edge region free of the contact layer. The regionsbeing free of the contact layer allow easier singulation along theseregions. In particular, uncontrolled tearing of the contact layer andassociated unintentional “fraying” of the contact layer duringsingulation can be prevented.

The contact layer may have at least one metallic layer, i.e. a layerwith one or more metals, a metal alloy and/or a metal mixture. Inparticular, the contact layer can have a stack of layers with severalmetallic layers. In this example, the contact layer is particularlypreferred to have an adhesive layer directly adjacent to the bottom sideof the substrate, for example, with or made of Ti. On an upper sidefacing away from the substrate, the contact layer can have a layer withor made of Au, which, for example, is intended and embodied as a bondlayer for wire bonding. In this example, the bond layer may preferablyhave a thickness of greater than or equal to 400 nm and less than orequal to 1.5 μm. In between, the contact layer may have one or moreother layers such as diffusion barrier layers, for example, with or madeof one or more materials selected from Pt, Pd, Ni, Cr and TiWN. If thecontact layer is provided for soldering the semiconductor laser, one ormore further layers, in particular, for example, a further diffusionbarrier layer with one or more of the aforementioned materials and/or afurther layer with or made of Au, which is thinner than the bond layerdescribed above, may be applied instead of or on the bond layerdescribed above. Local annealing in the at least one first partialregion can in particular result in an Au-containing layer becomingbrittle and thus no longer suitable for wire bonding or soldering. Asthe at least one second partial region remains unannealed, the at leastone second partial region can be used to connect the contact layer bybonding or soldering.

The bottom side of the substrate may have a surface structure. Inparticular, the surface structure may have a depression in the at leastone first partial region. The depression can, for example, be made byetching before the contact layer is applied. Thus, partial annealing ofthe contact layer can be combined with partial surface structures suchas those produced by etching. In this way, an additional operatingvoltage reduction can be achieved by reducing the substrate thickness inthese regions. If the at least one first partial region is located in anedge region of the bottom side of the substrate of the semiconductorlaser, an additional depression produced in this region can facilitateseparation.

A further layer of the contact layer may be applied after localannealing. The further layer can be applied to the at least one firstand/or second partial region. In particular, the second layer can beapplied over a large area, i.e. on the first and second partial regionsand can, for example, have Au or be made of Au. If the bottom side ofthe substrate has a surface structure, i.e. in particular one or moredepressions, the further layer can also be used for planarization.

The at least one first partial region may be applied at least partiallyin an edge region of the bottom side of the substrate of thesemiconductor laser in which a separation is carried out. In thisexample it can be advantageous if the substrate is also damaged by thelocal annealing, in particular by irradiation such that a fracturenucleation is initiated for singulating the wafer and/or a laser bar.

The semiconductor layer sequence of the semiconductor laser may have aridge waveguide structure. The ridge waveguide structure can inparticular be formed by a ridge-shaped, longitudinally extendingelevated area of the semiconductor layer sequence. The side surfacesbounding the ridge waveguide structure in the lateral direction can forma step profile, especially with the adjacent surface areas of the topside of the semiconductor layer sequence. The at least one first partialregion can run at least partly parallel to the ridge waveguide structureso that a current can be applied parallel to the ridge waveguidestructure. The at least one first partial region can at least partiallyoverlap with the ridge waveguide structure, especially when thesubstrate is viewed from the bottom side. This allows current imprintingfrom the substrate side as close as possible to the active region of theactive layer.

The partial annealing described here and thus the partial alloying ofthe contact layer has the advantage that the ohmic behavior of thecontact layer is improved compared to completely unannealed contactswithout the epitaxial electrode layer being thermally damaged. Overall,this results in a significant reduction in the operating voltagecompared to corresponding unannealed laser diodes, which is reflected inincreased efficiency. It is of essential importance for application ofthe described method that the preferably laser-supported annealingprocess, which only acts locally in the at least one first partialregion, leads to a corresponding local alloying process and thus not toa full-surface mixing of the contact layer on the bottom side of thesubstrate. This enables a subsequent low-loss electrical and thermalassembly of the component. In particular, the process can preventgallium from a Ga-containing substrate or titanium from the contactlayer from reaching the surface of the contact layer over a large areawhere it would lead to an electrical and thermal deterioration of thecomponent properties after unavoidable oxidation. The correspondingoxidation only takes place in one or more of the first partial regions,while the contact layer in one or more of the second partial regionsremains unaffected by annealing. Depending on the type of subsequentmounting of the semiconductor laser with the contact layer up or down ona heat sink, there are advantageous designs in which the alloying of thecontact layer takes place specifically only in one or more desired firstpartial regions of the contact layer. For example, if the semiconductorlaser is later mounted with the electrode layer on the epitaxial sidefacing downwards by soldering it onto a heat sink, a second partialregion of the contact layer can remain unannealed and thus be omittedfrom the alloying process intended for application of a bonding wire. Ina desired more homogeneous current application, e.g. for power laserswith high operating currents, several such second partial regions canalso be advantageous, whereby correspondingly more bonding wires can beused.

Another technologically advantageous combination is the integration ofadditional stress-relaxing structures in the partial annealing areasformed by the first partial regions. These relaxation structuresgenerated by etching or laser-induced material removal, for example, canbe advantageous for large-area chips or laser bars to reduce bowing ofthe component and thus enable a stable mounting with low loss and lowstress.

Further advantages, examples and developments are revealed by theexamples described below in connection with the figures.

In the examples and figures, identical, similar or identically actingelements are each provided with the same reference numerals. Theelements illustrated and their size ratios to one another should not beregarded as being to scale, but rather individual elements such as, forexample, layers, components, devices and regions, may have been madeexaggeratedly large to illustrate them better and/or to aidcomprehension.

FIGS. 1A to 1E show an example of a method of manufacturing asemiconductor laser diode 100, wherein FIGS. 1A and 1B show sectionalillustrations of a wafer composite. The area corresponding to a latersingulated semiconductor laser 100 is indicated in FIGS. 1A and 1B bythe dotted vertical lines. FIGS. 1C to 1E show views of a singulated andfinished semiconductor laser 100 in a view toward the light outcouplingsurface 6, a section through the semiconductor laser diode 100 with asection plane perpendicular to the light outcoupling surface 6, and aview toward the bottom side 10 of substrate 1.

As shown in FIG. 1A, a substrate 1 in the form of a wafer is providedfor the manufacturing of the semiconductor laser 100, which is, forexample, a growth substrate for a semiconductor layer sequence 2epitaxially grown thereon. Alternatively, the substrate 1 can also be acarrier substrate to which a semiconductor layer sequence 2 grown on agrowth substrate is transferred after growth. For example, the substrate1 may be made from GaN, on which a semiconductor layer sequence 2 basedon an InAlGaN compound semiconductor material is grown. In addition,other materials are also possible for the substrate 1 and thesemiconductor layer sequence 2. The semiconductor layer sequence 2 hasan active layer 3 that generates light 8 during operation, in particularlaser light when the laser threshold is exceeded, and emits it via afacet formed as a light outcoupling surface 6, as indicated in FIG. 1D.In addition to the active layer 3, the semiconductor layer sequence 2may contain further semiconductor layers such as cladding layers,waveguide layers, barrier layers, current spreading layers and/orcurrent limiting layers not shown to simplify the illustration. Inparticular, the semiconductor layer sequence 2 may have an n-doped and ap-doped side formed by respective semiconductor layers and between whichthe active layer 3 is arranged. As an example, the semiconductor layersequence 2 in the shown example is first grown on the substrate 1 withthe n-doped semiconductor layers, then with the active layer 3 and ontop of this with the p-doped layers. Alternatively, reverse polarity ofthe semiconductor layer sequence 2 on the substrate 1 may be possible,especially if the substrate 1 is formed by a carrier substrate.

On the side of the semiconductor layer sequence 2 facing away from thesubstrate 1, i.e. on the epitaxial side, an electrode layer 4 is appliedwhich is provided for electrical contacting of the semiconductor layersequence 2 from the epitaxial side. For example, the electrode layer 4may contain one or more of the following metals: Ag, Al, Au, Pt, Pd. Inthe top side of the semiconductor layer sequence 2 facing away from thesubstrate 1, a ridge waveguide structure 9 is formed by removing part ofthe semiconductor material from the side of the semiconductor layersequence 2 facing away from the substrate 1. The ridge waveguidestructure 9 runs in a longitudinal direction and is bounded on bothsides in the lateral direction by lateral surfaces. The ridge sidesurfaces and the remaining top side of the semiconductor layer sequence2 are covered by a passivation material 19, for example, an electricallyinsulating oxide, nitride or oxynitride with one or more materialsselected from Si, Al and Ti. Due to the refractive index jump at theside surfaces of the ridge waveguide structure 9 because of thetransition from the semiconductor material to the passivation material19, a so-called index guidance of the light produced in the active layer3 can be effected, which can substantially lead to formation of anactive region 5, which indicates the region in the semiconductor layersequence 2 in which the produced light is guided and amplified in laseroperation. Alternatively, the semiconductor laser diode 100 can also bedesigned as a so-called wide-strip laser diode without a ridge waveguidestructure.

Furthermore, after singulation of the wafer composite, reflective orpartially reflective or anti-reflective layers or layer sequences alsonot shown for the sake of clarity and provided and arranged forformation of an optical resonator in the semiconductor layer sequence 2,may be applied to the light outcoupling surface 6 and the opposite rearsurface 7, which form side surfaces of the semiconductor layer sequence2 and of the substrate 1 as shown in FIGS. 1C and 1D.

After the method step shown in FIG. 1A of providing the substrate 1having the semiconductor layer sequence 2 with the active layer 3, acontact layer 11 is deposited on the bottom side 10 of the substrate 1opposite the semiconductor layer sequence 2, as shown in FIG. 1B.Together with the electrode layer 4 on the semiconductor layer sequence2, the contact layer 11, during operation of the semiconductor laser100, injects current into the semiconductor layer sequence 2 and inparticular into the active layer 3. The contact layer 11 has at leastone first partial region 12 and at least one second partial region 13,which are continuously formed. The at least one first partial region 12and the at least one second partial region 13 form a continuous part ofthe contact layer 11 and directly adjoin each other in a lateraldirection. In particular, the first contact layer 11 is applied over alarge area and all first and second partial regions 12, 13 have,immediately after application, the same structure and composition. If anedge region of the later singulated semiconductor laser 100, as shown insome examples below, is to be free of the contact layer 11, the contactlayer can still be structured accordingly in the wafer composite so thatthe bottom side 10 is free of the contact layer 11 at the boundaries ofadjacent semiconductor lasers, i.e. in the region of the vertical dashedlines drawn in FIGS. 1A and 1B.

In the example shown, the contact layer 11 has a single layer orpreferably a layer structure with metallic layers. In particular, thecontact layer 11 may comprise, directly on the bottom side 10 ofsubstrate 1, an adhesion promoting layer, for example, or with Ti,thereover one or more diffusion barrier layers, for example, or with oneor more materials selected from Pt, Pd, Ni, Cr and TiWN, and thereover alayer of or with Au. When the semiconductor laser 100 with the electrodelayer 4 is soldered to a heat sink and electrically connected to thecontact layer 11 by a bonding wire, the uppermost layer of the contactlayer 11 with or of Au may have a thickness of preferably greater thanor equal to 400 nm and less than or equal to 1.5 p.m. If thesemiconductor laser 100 is to be soldered with the contact layer 11 ontoa heat sink, the contact layer can have, on or instead of the describedAu layer, one or more further diffusion barrier layers and a furtherlayer made of or with Au, which has a smaller thickness.

After application, the contact layer 11 is annealed by irradiation 90 inat least one first partial region 12, as indicated in FIG. 1B. Inparticular, the contact layer 11 is annealed only in the at least onefirst partial region 12, while the contact layer 11 is annealed in nosecond partial region 13. The irradiation 90, which is particularlypreferably provided by a light source, causes a local temperatureincrease in the at least one first partial region 12, wherein in the atleast one first partial region 12 materials of the contact layer 11 andthe substrate 1 are mixed. Annealing can thus cause alloying of at leastpart of the contact layer 11 in the at least one first partial region12, while the contact layer 11 remains unchanged in the at least onesecond partial region 13 and no material mixing and thus no alloyingtakes place there. The at least one second partial region 13 remainsunannealed especially in the finished semiconductor laser 100. Theirradiation is preferably effected by laser light which, for example,can cover the at least one first partial region 12 by scanning, andparticularly preferably comprises a wavelength at least partiallyabsorbed by the substrate 1.

By local annealing only in the at least one first partial region 12, inparticular the layer structure of the contact layer 11 is changed thereand a mixture of materials at the interface between the substrate 1 andthe contact layer 11 is produced, as described above, by at leastpartial melting of the respective materials. This leads to a significantimprovement in the flux voltage of the semiconductor laser 100, as wedemonstrated experimentally several times. Furthermore, local annealingin the at least one partial region 12 may result in embrittlement of theAu-containing layer and migration of Ti from the adhesion promotinglayer and/or Ga from the substrate 1 to the surface and in oxidationthere. As a result, the contact layer 11 is no longer easily bondable orsolderable in the at least one first partial region 12, but has the lowcontact resistance described above. In the unannealed at least onesecond partial region 13, however, the contact layer 11 keeps itsdesired structure and its good bondability or solderability. In theshown example, the first partial region 12 runs between the facets in astrip parallel to the ridge waveguide structure 9, i.e. from the lightoutcoupling surface 6 to the rear surface 7, and overlaps with the ridgewaveguide structure 9 in a view along the growth direction of thesemiconductor layer sequence 2. In other words, the first partial region12 is arranged below the ridge waveguide structure 9 so that theshortest possible current path in the semiconductor layer sequence 2during operation is achieved.

After local annealing, the wafer composite is singulated intosemiconductor lasers 100, one of which is shown in FIGS. 1C to 1E. Inthe shown example, the contact layer 11 in the singulated semiconductorlaser 100 has an annealed strip-shaped first partial region 12, whichruns purely exemplarily parallel to and overlaps the ridge waveguidestructure 9 and is arranged between two unannealed second partialregions 13. Due to the contact layer 11, which is only partiallyalloyed, the semiconductor laser 100 exhibits a reduced flux voltage,which leads to a better component efficiency and thus also to animproved component stability. The unalloyed regions of the contact layer11, on the other hand, ensure good mountability as described above for areliable, low-loss and thermally controlled soldering or bondingprocess.

The form, size and number of the first and second partial regions 12, 13shown in FIGS. 1A to 1E are to be understood as examples only. Inparticular, the contact layer 11 may have a plurality of first and/or aplurality of second partial regions 12, 13 in different geometricarrangements. For example, a first partial region 12 may have one ormore geometric shapes that are selected from: line, cross, circle,ellipse, spiral, grid, square, wavy line, meander and combinationsthereof. In the following figures, further examples of the semiconductorlaser 100 are shown in a representation corresponding to FIG. 1E, unlessotherwise described, which shows contact layers 11 with a large numberof possible variations of the first and second partial regions 12, 13.In particular, the shown different variations of the first and secondpartial regions 12, 13 of the contact layers 11 can also be combinedwith each other.

The semiconductor laser 100 shown in FIG. 2A has a contact layer 11which, as in the previous example, is applied over the entire surface ofthe bottom side of the substrate and completely covers it. In contrastto the previous example, however, the contact layer 11 has two firstpartial regions 12 between which, in the lateral direction, a secondpartial region 13 is arranged. For better illustration, the purelyexemplary position of the ridge waveguide structure 9 is also indicatedby the dotted area. The first two partial regions 12 are arranged inlateral edge regions of the bottom side of the substrate. This may allowhomogeneous current imprinting from the substrate edges, while thesecond partial region 13 in between is intended for electricalconnection by a bonding wire or for assembly by soldering. Furthermore,it can be advantageous if the substrate is also damaged by the localannealing in the first partial regions 12, in particular by theirradiation, that a fracture nucleation for singulation is formed as aresult. The structure shown in FIG. 2A can also be denoted as aso-called scribing structure. The same advantage of combining an alloystructure with fracture nucleations for improved singulation can also befound in the following examples with first partial regions 12 in edgeregions of the bottom side of the substrate.

FIG. 2B shows an example in which the bottom side 10 of substrate 1 isfree of the contact layer 11 in an edge region. In particular, thesemiconductor laser 100 has a laterally circumferential edge region ofthe bottom side 10 which is free of the contact layer 11. Such astructured contact layer 11 can be advantageous with regard tosingulation since uncontrolled tearing of the contact layer 11 can beavoided, for example, during facet breaking during the singulationprocess.

FIG. 3A shows an example for a contact layer 11, where the arrangementof a first partial region 12 along the ridge waveguide structure 9 andbetween two second partial regions 13 corresponds to the exampledescribed in FIGS. 1A to 1E. In this example, the shortest possiblecurrent path in the semiconductor layer sequence is achieved. Incontrast to the example of FIGS. 1A to 1E, however, as described inconnection with the example of FIG. 2B, a circumferential edge region ofthe bottom side 10 of the substrate 1 is free of the contact layer 11.

In the examples shown in FIGS. 3B and 3C, the contact layer 11 hasseveral first partial regions 12 parallel to the ridge waveguidestructure 9 and overlapping it, separated from each other by acontinuous second partial region 13. An improvement of the transverseconductivity of the contact layer 11 can be achieved by the continuoussecond partial region 13.

FIG. 4A shows an example of a contact layer 11 with a cross-shaped firstpartial region 12 separating four second partial regions 13 from eachother. For example, each of the second partial regions 13 can beprovided for the connection of a bonding wire, which allows a highcurrent injection to be achieved, for example, for high currentapplications. FIG. 4B shows an example in which the first partial region12 is additionally formed in a circumferential edge region to improvethe electrical connection of the contact layer 11 to the substrate. Theexample illustrated in FIG. 4C shows, compared to the two previousexamples, six second partial regions 13 separated from each other by thecontinuous first partial region 12. Alternatively, more or fewer secondpartial regions 13 are also possible.

The example shown in FIG. 4D shows, compared to the previous examples, aplurality of first partial regions 12 each parallel to the ridgewaveguide structure 9 and arranged in two groups along the beamdirection. Each of the groups of the first partial regions 12 has aplurality of the first partial regions 12 in a direction perpendicularto the direction of radiation. The second partial region 13 is formedcontinuously. In the direction of radiation between the lightoutcoupling surface and one of the groups of first partial regions 12,between the two groups of partial regions 12 and between the rear sideand the other of the two groups, the second partial region 13 has one ormore regions each provided for connection of a bonding wire, asindicated by the dotted lines. This allows a high current injection forhigh-performance diodes to be achieved. The shown number, size anddensity of the first partial regions 12 and the groups of the firstpartial regions 12 is to be understood to be only exemplary and can alsovary depending on the requirements with regard to the semiconductorlaser diode.

The examples shown in FIGS. 5A to 5C correspond to the examplesdescribed in connection with FIGS. 4A to 4C, wherein the second partialregions 13 in FIGS. 4A to 4C, which are separated from each other, areeach formed as a continuous second partial region 13 in the examples ofFIGS. 5A to 5C to improve the transverse conductivity of the contactlayer 11.

FIG. 6A shows an example of a contact layer 11 having a first partialregion 12 in a laterally circumferential edge portion of the bottom sideof the substrate that completely surrounds a second partial region 13 ina lateral direction. In the example of FIG. 6B, in contrast, the regionat the facets is omitted in which the ridge waveguide structure isarranged. The contact layer 11 of the example shown in FIG. 6C, on theother hand, has a first partial region 12 circumferential along theedge, which additionally has finger-like structures protruding into acentral region to enable a more homogeneous electrical connection of thecontact layer 11 to the substrate.

The examples shown in FIGS. 7A to 7C and FIGS. 8A to 8C haveserpentine-like or meander-shaped as well as spiral-shaped first partialregions 12. In FIGS. 7A and 8A, the bottom side 10 of the substrate 1 isfree of the contact layer 11 in a circumferential edge region, while thecontact layers 11 of the examples of FIGS. 7B and 8B are applied overthe entire surface. The examples shown in FIGS. 7C and 8C showadditional first partial regions 12 at the edges in the form of ascribing structure for easier singulation described above in connectionwith FIG. 2A.

FIGS. 9A and 9B show contact layers 11 applied with a freecircumferential edge region (FIG. 9A) or over the entire surface (FIG.9B) and have circular first partial regions 12 separated from each otherby corresponding second partial regions 13. FIG. 9C shows an example inwhich the first circular partial regions 12 are open so that acontinuous second partial region 13 is formed to improve the transverseconductivity of the contact layer 11.

While the semiconductor lasers 100 described in the previous examplesare designed as single emitters, semiconductor lasers 100 designed aslaser bars are shown in FIGS. 10A to 10D, each of which has a pluralityof ridge waveguide structures 9 and thus a plurality of active areasthat emit laser light. The number of ridge waveguide structures 9 inFIGS. 10A to 10D is to be understood purely as an example and canpreferably be greater than or equal to 2 and less than or equal to 50. Acontact layer 11 can be applied to the bottom side of the substrate in acontinuous manner in each example, the contact layer 11 having a firstpartial region 12 being circumferential and, purely as an example,between adjacent active regions (FIG. 10A) or pairs of adjacent activeregions (FIG. 10B). As shown in FIG. 10C, the first partial regions 12can also extend transversally across the active areas. Furthermore, thefirst partial regions 12 can also be arranged between the active areasas shown in FIG. 10D. As an alternative to the shown shapes of the firstand second partial regions 12, 13, they can also be designed asdescribed in the other examples.

FIG. 11A shows a semiconductor laser 100 according to a further examplein a view toward the contact layer 11 and in a section of a sectionalview along the indicated sectional plane AA, in which the substrate 1has a surface structure 20 on the bottom side 10 which is formed as adepression. As an example, the contact layer 11 is embodied with regardto the first and second partial regions 12, 13 as described above inconnection with FIG. 4A. Alternatively, the surface structure 20 canalso be combined with any other example. The contact layer 11 applied onthe bottom side 10 has at least one first partial region 12 which isarranged in the depression. In particular, the at least one firstpartial region 12 can cover the entire depression as shown in theexample. In other words, wherever the first partial region 12 of thecontact layer 11 is formed, there is also a depression in the bottomside 10 of the substrate 1. The surface structure 20 can be formed, withregard to the course of one or more depressions such as the firstpartial regions 12, as shown in connection with the examples describedabove.

The surface structure can be produced, for example, by etching or laserablation before the contact layer 11 is applied to the bottom side 10 ofthe substrate 1. During partial annealing of the contact layer 11, thebottom side 10 can then be scanned by laser irradiation along thesurface structure 20 so that the depression or depressions is/arecovered with one or more first partial regions 12. The surface structure20, for example, can lead to an increase in the contact area between thefirst partial region 12 and the substrate 1 and to a reduction in thesubstrate thickness in the area of the first partial region 12, whichcan lead to a lower electrical resistance of the semiconductor laser100. Furthermore, the surface structure can lead to an improvement withregard to tensions due to the combination of alloy- and tension-relaxingstructures.

In FIG. 11B a semiconductor laser 100 is shown according to an examplein a view on the contact layer 11 and in a section of a sectional viewalong the sectional plane AA. Compared to the example in FIG. 11A, thecontact layer 11 in the example in FIG. 11B has a further layer 14 overthe first and second partial regions 12, 13 applied after localannealing. Also, the further layer 14 can be applied only to the atleast one first or second partial region 12, 13. As shown, the furtherlayer 14 can be applied over a large area, i.e. on all first and secondpartial regions 12, 13, and can, for example, have Au or be made of Au.If the bottom side 10 of the substrate 1 has a surface structure 20,i.e. in particular one or more depressions, as in the shown example, thefurther layer 14 can also serve in particular for planarization, whichlevels out the surface structure 20. The further layer 14 can also bepart of the contact layers 11 of all the other described examples.

FIG. 11C shows a semiconductor laser 100 according to a further examplein a view toward the contact layer 11 and in a section of a sectionalview along the drawn sectional plane AA, in which the surface structure20 has a depression in an edge region of the bottom side 10 of thesubstrate 1. At the same time, the contact layer 11 in this edge regionis locally annealed so that a first partial region 12 is formed in theedge region. As an example, the contact layer 11 is embodied with regardto the first and second partial regions 12, 13 as described above inconnection with FIG. 6B. Alternatively, the surface structure 20 can becombined with a depression at the edge or with any other example. Thetrench forming the depression combined with the locally alloyed firstpartial region 12 at the chip edge covering the depression allows notonly an improved electrical contact between the contact layer 11 and thesubstrate 1, but also an improved singulation process, in particular bybreaking the substrate 1 in the region of the depression at the edge. Inparticular, this can result in an improved facet fracture quality.

FIGS. 12A to 12C show images of sections of a semiconductor laserembodied purely exemplarily as the example described above in connectionwith FIG. 2A. The locally annealed first partial regions 12 andunannealed second partial regions 13 of the contact layer 11 are clearlyvisible.

The examples illustrated in the figures can also be combined with oneanother, even if not all such combinations are explicitly shown.Furthermore, the examples shown in the figures may have additionaland/or alternative features according to the description in the generalpart.

Our lasers and methods are not limited by the description based on theexamples. Rather, this disclosure includes each new feature and eachcombination of features, which includes in particular each combinationof features in the appended claims, even if the feature or combinationitself is not explicitly explained in the claims or examples.

This application claims priority of DE 10 2016 120 685.7, the subjectmatter of which is incorporated herein by reference.

What is claimed is:
 1. A semiconductor laser comprising: a substratehaving a semiconductor layer sequence with an active layer thatgenerates light during operation of the semiconductor laser, and acontact layer on a bottom side of the substrate opposite thesemiconductor layer sequence, wherein the contact layer has at least onefirst partial region and at least one second partial region which areformed contiguously, the at least one first partial region is annealed,and the at least one second partial region is unannealed.
 2. Thesemiconductor laser according to claim 1, wherein the contact layer isarranged on an entire surface of the bottom side of the substrate. 3.The semiconductor laser according to claim 1, wherein the contact layercovers an entire surface of the bottom side of the substrate except foran edge region.
 4. The semiconductor laser according to claim 1, whereinthe contact layer comprises a plurality of layers.
 5. The semiconductorlaser according to claim 1, wherein a further layer is applied at leastto the first partial region.
 6. The semiconductor laser according toclaim 1, wherein the bottom side of the substrate has a surfacestructure comprising a depression in the at least one first partialregion.
 7. The semiconductor laser according to claim 6, wherein the atleast one first partial region of the contact layer is arranged in thedepression.
 8. The semiconductor laser according to claim 1, wherein thecontact layer has a plurality of first partial regions separated fromone another by one or more second partial regions that are locallyannealed.
 9. The semiconductor laser according to claim 1, wherein theat least one first partial region is arranged in an edge region of thefirst contact layer.
 10. The semiconductor laser according to claim 1,wherein the semiconductor layer sequence comprises a ridge waveguidestructure and the at least one first partial region extends parallel tothe ridge waveguide structure.
 11. The semiconductor laser according toclaim 10, wherein the at least one first partial region overlaps withthe ridge waveguide structure when viewed from the bottom side of thesubstrate.
 12. The semiconductor laser according to claim 1, wherein theat least one first partial region has one or more geometric shapesselected from the group consisting of line, cross, circle, ellipse,spiral, grid, square, wavy line and meander.
 13. The semiconductor laseraccording to claim 1, wherein the substrate is electrically conductive.14. The semiconductor laser according to claim 1, wherein the substratecontains Ga.
 15. The semiconductor laser according to claim 1, whereinthe first partial regions comprise stress-relaxing structures.
 16. Thesemiconductor laser according to claim 1, wherein the contact layercomprises an adhesion promoting layer directly on the bottom side of thesubstrate.
 17. The semiconductor laser according to claim 16, whereinthe adhesion promoting layer comprises Ti.
 18. The semiconductor laseraccording to claim 1, wherein the contact layer comprises one or morediffusion barrier layers comprising one or more materials selected fromthe group consisting of Pt, Pd, Ni, Cr and TiWN.